Wide bandwidth radar having improved signal to clutter response characteristics

ABSTRACT

Improved signal to clutter response in a radar is achieved by transmitting broad bandwidth frequency modulated noise pulses. Utilization of millisecond pulse intervals enables the radar video processor to average the independent samples present in each echo pulse so that each pulse represents an estimate of the true average return from the background. The bandwidth of the IF processing circuit is equal to the RF circuit and transmitted pulse bandwidths.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

BACKGROUND OF THE INVENTION

This invention relates to radars used to detect and track targets inclutter backgrounds and in particular to a wide bandwidth radar of thattype having improved signal to clutter response characteristics.

A need exists to extend the present capability of guided weapons toinclude all-weather air strike operations against tactical targets.Microwave radiometric guidance techniques provide the potential tostrike targets partially obscured by foliage, clouds, or adverseweather. Trackers utilizing these techniques are capable of operating inboth active and passive modes.

Past work has shown that passive microwave radiometer techniques can beused to detect and track metal targets in foliage backgrounds even whenpartially obscured by weather, foliage and camouflage. However, for mosttactical warfare applications, detection range is not sufficient andfalse targets such as pools of water and background variations are asignificant problem.

With regard to active mode operation, a class of frequency-agile radarhas evolved which uses pulse-to-pulse frequency change to smooth clutterfluctuations. Typically, using spin-tuned magnetrons, these radarsachieve detection ranges of greater than 10 miles against targets in seaand terrain backgrounds. In this class of radar the frequency of thetransmitted pulse is changed from pulse to pulse and a narrowbandreceiver is programmed to follow the frequency variation. The primaryproblem with this approach is the complexity/cost of the frequency agiletransmitter and receiver.

A second active mode approach is to use an FM-CW radar with widefrequency deviation. With this approach, range gating is difficultbecause sweep linearity must be obtained over a very large frequencyrange. Also, since it is a CW system, feedthrough of transmitter noisesidebands into the receiver limits system performance.

The present invention relates to active mode operation and specificallyto a class of active radars using wide bandwidth to suppress clutterfluctuations from terrain and sea backgrounds and to suppress signalfluctuations and angular glint from complex targets such as tanks. Thussignal-to-clutter ratios are increased such that search, acquisition andguidance can be accomplished for small targets such as tanks in terrainbackground and small ship/boats at sea.

Compared to the magnetrons used in frequency-agile systems, the solidstate transmitter power is very low. However, since radar range has a4th root dependence on power, ranges usable in missile terminal guidancesystems are achievable. The major advantage of the pulsed, widebandwidth radar over the FM-CW radar approach is the ability to use timegating to discriminate against rain backscatter, system thermal noise,and false or multiple targets.

SUMMARY OF THE INVENTION

The invention comprehends a wide bandwidth radar having improved signalto clutter response characteristics. The approach used to reducebackground clutter fluctuations is to chirp each radar pulse by severalhundred megahertz. The bandwidth of the receiver IF is made equal to thetransmitted bandwidth. Averaging of the clutter fluctuations isacccmplished by the video detector and filter. Each video pulse gives anestimate of the non-fluctuating, average radar cross section of thescene being illuminated.

In one embodiment the technique of the invention is implemented byutilizing a K_(a) -band IMPATT noise generator to generate a train ofone millisecond 500 MHz bandwidth, pulses. The radar RF and IF circuitsalso have 500 HMz bandwidths. A 250 MHz bandwidth video circuit averagesthe content of each return pulse to provide an estimate of the trueaverage return from background.

It is a principal object of the invention to provide a radar havingimproved signal to clutter response characteristics.

It is another object of the invention to provide a radar capable ofdetecting targets in the face of sea or terrain background clutterfluctuations.

It is another object of the invention to provide an active radar withimproved signal to clutter response having greater detection range thanpassive mode systems.

It is another objet of the invention to provide an active radar withimproved signal to clutter response and having the ability to utilizetime gating to discriminate against rain backscatter, system thermalnoise and false multiple targets.

These together with other objects, features and advantages of theinvention will become more readily apparent from the following detaileddescription when taken in conjunction with the illustrative embodimentsin the accompanying drawing.

DESCRIPTION OF THE DRAWING

FIG. 1 is a functional block diagram of a polarization agile radar thatincorporates one presently preferred embodiment of the invention;

FIG. 2 is a graph illustrating frequency modulation of the type IMPATToscillator utilized in implementing the invention;

FIG. 3 is a schematic diagram of the IMPATT modulator utilized inimplementing the invention; and

FIG. 4 is a block diagram of the IF circuit of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In a conventional narrowband radar a pulse is transmitted from theantenna and illuminates the target and a background resolution elementas defined by the angular beamwidth of the antenna and the pulsewidth.The return pulse is downconverted in a mixer to an intermedite frequency(IF) and the envelope of the pulse is detected and amplified. Rangegating and other video processing is performed on the video pulse train.The IF amplifier bandwidth is approximately the reciprocal of thepulsewidth to obtain maximum receiver sensitivity along with acceptablepulse shape reproduction. The video amplifier bandwidth is approximatelyone-half of the reciprocal of the pulsewidth, or half the IF bandwidth,also to maintain acceptable pulse shape reproduction. If the radar moveswith respect to the background or if the background moves (windblownfoliage), the return from the background fluctuates from pulse to pulsebecause of the random change in phase of return from various parts ofthe background. In a conventional frequency agile radar the frequency ofsuccessive pulses is changed, in steps sufficient to change the phasingof returns from various parts of the terrain, an amount sufficient toobtain an independent sample of the fluctuation. Returns from a numberof pulses are then averaged to obtain an estimate of the average returnsignal amplitude.

In the wide bandwidth radar technique which is the subject of thepresent invention, each transmitted pulse is frequency modulated orchirped so as to obtain a broad power spectral density. The transmittedpulse illuminates and is reflected from the target and the backgroundpatch. The effect of the wide bandwidth modulation on each pulse returnfrom the background is to create a randomly varying amplitude during thepulse interval, which represents a number of independent samples of theclutter fluctuations. An RF and IF bandwidth equal to the transmittedbandwidth is needed to pass the high frequency variations during thepulse interval. The effect of the video detector is to average theindependent samples present during each pulse interval so that eachpulse represents an estimate of the true average return from thebackground. The video amplifier bandwidth need be only wide enough topass the pulse shape.

Referring now to FIG. 1 there is illustrated thereby a functional blockdiagram of a radar that incorporates the present invention. By way ofshowing a complete radar system certain conventional radar circuits andother features not germane to the present invention are included in theblock diagram. However, in general, the following detailed descriptionis limited to those circuits and components necessary to understand andpractice the invention.

The Polarization Agile Tracking radar shown in FIG. 1 operates at K_(a)-band. Both active and passive modes are provided. Either linear orcircular polarization agility can be selected for both modes. In theactive mode which relates to the presently described embodiment of theinvention the target is illuminated by a 1.0-microsecond pulse from anIMPATT transmitter whose frequency is swept over a 500-MHz bandwidth.The wide bandwidth suppresses the background clutter fluctuations,thereby enhancing the return signal-to-clutter (S/C) ratio. In thepassive mode, the receiver is a wideband Dicke radiometer with a 5-kHzswitching frequency that alternately selects and processes theorthogonal components of either linear or circular polarization. In bothactive and passive modes, closed-loop tracking signals are obtained byphase comparison of the received conical-scan modulated signal with asine-cosine reference generator attached to the antenna spin motor.

The functional block diagram of the Polarization Agile Radiometer shownin FIG. 1 is considered in terms of main functional areas: the RFassembly, including the antenna 11, transmitter 12, receiver mixer 13,local oscillator (LO) 14, polarization switching unit 15, and other RFcomponents; the IF assembly containing the IF amplifiers 17, 18, AGCattenuator 22, switched attenuator 19, and detector/video preamplifier20, 21; the electronics circuitry including video and scan rateamplifiers 24, 25, Dicke rate amplifier 26, clamp circuit, synchronousdetector, filters, and phase detectors; and, the azimuth and elevationservo tracking loop components.

In the passive mode, RF energy is received from the target by theconical-scan antenna, passes through the polarization switching unit,and is converted to the 50- to 550-MHz IF band by the balanced mixer.The polarization switching unit alternately connects the receiver to thetemperature stabilized RF load at a 5-kHz rate and to each of theselected orthogonal polarization components at a 2.5-kHz rate. Thereference and wide band received signals are amplified in the IFamplifier, and their amplitude which is modulated by the conical-scanantenna is detected by the detector. The 5-kHz Dicke switched signalsare amplified further in the video and Dicke rate amplifiers. Followingrestoration of the waveform dc level and synchronous detection theorthogonally polarized received signals are separated and filtered toproduce the radiometer temperature measurement output. After theorthogonally polarized signals are separated, tracking signals areobtained by amplifying the conical-scan modulation in the scan rateamplifiers. A polarization track switch is provided to select thedesired polarization ccmponent for closing the tracking loop through thephase detectors.

In the active mode the target is illuminated by the transmitter at a PRFof 5 kHz. The received pulse train, whose amplitude varies with theconical-scan modulation, is converted to IF, amplified, detected, andthe conical-scan tracking modulation extracted for comparison in thephase detectors. Essentially the same receiver RF and IF components areused in the active and passive modes. The major difference between thetwo is the switching and gating that is accomplished to isolate thetransmitter and receiver.

The basic timing pulses for the transmitter and receiver are produced bya 5-kHz oscillator 30 in the timing unit 29. To blank the receiverduring the transmit pulse, the timing unit sends a trigger pulse to thepolarization switching unit 15 and switched attenuator. During theblanking pulse the polarization switching unit switches the receiver tothe temperature stabilized load, thereby introducing 20 dB of additionalisolation to protect the mixer crystals. At the same time, the timingunit switches the switched IF attenuator 19 into the high attenuationstate to blank the detector and video circuits. After the receiver isblanked, a delayed trigger pulse causes the modulator 32 to pulse theIMPATT transmitter 12.

The K_(a) -band IMPATT oscillator produces a 1-microsecond wide burst ofnoise having a bandwidth of approximately 500 MHz. This pulse passesthrough the polarization switching unit and is radiated by the antenna11.

The transmitter leakage that enters the balanced mixer will cause thelast IF amplifier to saturate. The switched attenuator prevents thesaturated output from reaching the detector.

After the transmitter pulse occurs, the polarization switching unitconnects the receiver to the selected polarization port. The saturationrecovery time of the IF amplifiers 17, 18 is less than 1 microsecondwhich, together with the switching time of the polarization switchingunit and switched attenuator, results in a minimum receiver recoverytime of about 1.5 microseconds.

At close range, large target returns are maintained at a constant levelat the output of the video preamplifier 24 by the AGC circuit. The AGCattenuator provides an AGC range in excess of 50 dB.

The antenna is an 11-inch diameter Cassegrain that is con-scanned byrotating a tilted subreflector. A single spar supports the spin motor 36which rotates at 8000 RPM. The sine-cosine phase reference required toprocess the received conical-scan modulated signal is provided by thereference generator 25 attached to the rear shaft of the motor. Asolenoid located directly behind the dish rotates a polarizing sectionof the circular waveguide feed to produce either circular or linearpolarization.

The K_(a) -band IMPATT Transmitter 12 is a solid state IMPATT oscillatorthat is pulsed on by a transistorized modulator. The transmitteroperates at PRF of 5 kHz and a pulsewidth of 1.0 microsecond. The peakpower output is about 1 watt, with the 0.005 duty cycle resulting in anaverage power of 5 milliwatts.

Load variations and reflections are prevented from affecting theoscillator by using an isolator IF between the transmitter and thepolarization switching unit. A 20-dB directional coupler 39 and crystaldetector 40 are used to monitor transmitter power output and pulseshape.

FIG. 2 shows how the transmitted frequency of three different IMPATTdiodes changes during the pulse. The bandwidth of the transmited pulsefrom each diode is greater than 500 MHz. The first 100 ns of thetransmitted pulse is very noisy. Then the frequency tends to chirpdownward and stabilizes at the end of the pulse.

The modulator 32 provides the driving pulse for the IMPATT oscillator12. Power for the modulator is obtained from the +28 vdc supply and froma +70 vdc solid-state dc-to-dc power supply (not shown). The schematicdiagram of the modulator is shown in FIG. 3.

In operation, a +5 volt transmit trigger pulse from Timing Unit 29 isapplied to the base of Q8 which is used to provide immunity againstinput noise transients that are less than 1.5 volts. The negative goingpulse at the collector of Q8 is inverted by Q7 and used to trigger the9601 monostable multivibrator (A2). The width of the resulting +3 voltpulse at pin 8 is set for 1.0 microsecond by adjusting R2.

Additional noise-trigger immunity is provided by the monostablemultivibrator A4 which produces a gating pulse to prevent A2 from beingretriggered within 100 microseconds. This limits the modulator pulserate to 10 kHz to protect the IMPATT diode from burn-out.

The positive pulse at the output of the monostable multivibrator (A2) isapplied to the strobe input (pin 10) of the DM7820 line receiver (A3).Because both inputs (pins 11 and 13) are high, the strobe causes theoutput at pin 8 to change from a high to a low. This turns on Q1 sinceits base is held at a constant +5.0 vdc by the LM109H voltage regulator(A1). Turning on Q1 causes collector current to flow, putting anegative-going pulse on the base of Q2 causing it to conduct. Thevoltage at the base of the parallel-connected 2N3019 transistors risesto approximately 40 volts. This applies a 40-volt, 1.6-ampere pulse tothe IMPATT oscillator. The pulse current is maintained constant by thecurrent feedback action of Q2. The output frequency of the IMPATToscillator can be varied by adjusting the pulse current with R6. Turningthe adjustment screw clockwise increases the current.

Since the modulator output circuit is extremely rugged, theparallel-connected transistors can drive a short circuit load withoutdamage.

The Gunn LO 14 is a small solid-state device that provides theoscillator signal for the balanced mixer. It is mechanically tuned toF_(o) GHz and has a power output of 18 milliwatts. A resistiveattenuator reduces the output power to a 3.0-milliwatt level to drivethe mixer diodes. In addition to the resistive attenuator, an isolatoris provided between the oscillator and mixer to supply additionaldecoupling.

The Gunn LO operates from a 5.0-volt, 500-milliampere regulated powersupply.

The Mixer and IF Preamplifier are combined in an integral unit thatconverts the received RF signal into the 50- to 550-MHz IF band. In themixer 13, two matched diodes in separate holders are driven from an HPlane Tee to form a balanced configuration for cancelling LO noise. Thediode holders are fabricated by an electroforming process that enablesthe inside dimensions to be precisely controlled. Gold plating is usedon the inside surfaces to reduce loss. The IF output is brought out ofeach mixer through a low capacity RF choke.

The IF Preamplifier is noise-matched to the mixer IF output. Its gain is16 dB over the 50 to 550 MHz band with a maximum noise figure of 2.5 dB.

The measured double sideband noise figure of the Mixer and IFPreamplifier is 7.5 dB with an LO drive level of 3.0 milliwatts.

The IF Assembly is illustrated in block diagram form in FIG. 4. Itincludes the IF post-amplifier 47, AGC attenuator 49, IF amplifier 50,switched attenuator 51, and the detector/video preamp 52. All units areseparate, fully shielded, and interconnected by semirigid coaxial. Thisassembly amplifies the IF sigals, provides the switching and AGC actionnecessary to keep transmitter leakage and strong signal returns fromsaturating the video circuits, and detects and amplifies theconical-scan modulation on the return signal.

The first IF amplifier 17 consists of a preamplifier that is in integralcomponent of the mixer and in IF Post-Amplifier located on the IFassembly. The IF Post-Amplifier increases the IF signal level by 20 dBto prevent the AGC attenuator from affecting the receiver noise figure.

The AGC Attenuator is used in the active mode to prevent large, close-intarget returns from saturating the second IF amplifier and videocircuits. It is a passive device using PIN diodes. Insertion loss istypically 1.8 dB, and the attenuation changes linearly over a 40-dBrange as the AGC voltage is varied from 1 to 5 volts. Speed of responseis on the order of 10 dB per microsecond.

An additional 30 dB of gain is provided by the second IF amplifier 18.This raises the noise power of the radiometer to a value that issufficient to operate the detector. In the active mode, leakage from thetransmitted pulse is allowed to saturate this amplifier. Its recoverytime is designed to be less than 1 microsecond to maintain a minimumrange capability of 1000 feet.

The Switched Attenuator is only used in the Active Mode. It prevents thetransmitter leakage pulse from entering the IF amplifier and detector.Its operation is controlled by the timing unit which turns it on at theend of the transmitter pulse

The Switched Attenuator is a double-balanced mixer that is operated asan attenuator. It is turned on by inserting a dc control current intothe I port. Insertion loss in the ON state is approximately 2 dB, and byremoving the current, 35 to 40 dB of attenuation is obtained. Theattenuator can be switched on or off in 10 nanoseconds.

The detector and video preamplifier are integrated together in a singleunit. The detector is operated in the square-law region in both theActive and Passive Modes. Tangential sensitivity of the detector is onthe order of -53 dBm for the 5-MHz video bandwidth. The detectoroperates in the square law region at input power levels up to -10 dBm.This results in a possible 40-dB dynamic range for square law operation.

The video pulse train can be processed using conventional videotechniques such a pulse integration and range gating. Observing thebandwidth limitations stated, the technique can be used at any frequencyband and in either monopulse or conical scan radars.

The technique is not restricted with regard to RF frequency of operationbut performance depends on the implementation (power, bandwidth, antennabeamwidth, etc). The technique is usable in both conical scan andmonopulse trackers.

The key advantages of the invention are that it obtains clutterfluctuation reduction without the complexity of a conventional frequencyagile radar, and can be much more easily range gated than an FM-CWradar. Range gating is needed to discriminate against rain backscatterand limit the size of the background patch in order to discriminateagainst background variation and false or multiple targets. For missileseeker applications, this approach results in the least possible complexradar implementation with adequate performance.

While the invention has been described in its preferred embodiment it isunderstood that the words which have been used are words of descriptionrather than words of limitation and that changes within the purview ofthe appended claims may be made without departing from the scope andspirit of the invention in its broader aspects.

What is claimed is:
 1. A wide bandwidth radar having improved signal toclutter response comprisinga radar signal pulse generating meansgenerating a train of wide bandwidth non-coherent pulses, modulationmeans effecting wide bandwidth frequency modulation of said non-coherentpulses, transmit/receive means transmitting said train of frequencymodulated pulses and receiving echo signals thereof, a wide bandwidth IFcircuit providing IF processing of said received echo signals, and avideo circuit including a detector means receiving said IF processedecho signals said detector means being effective to average the contentsof each pulse thereof whereby each averaged pulse represents an estimateof the true average return from background clutter.
 2. A wide bandwidthradar as defined in claim 1 wherein the bandwidth of said transmittedpulses and the bandwidth of said IF processing circuit are substantiallyequal.
 3. A wide bandwidth radar as defined in claim 2 wherein saidmodulator means effects a broad power spectral density, in saidtransmitted pulses and a randomly varying amplitude during the pulseinterval of each echo signal pulse.
 4. A wide bandwidth radar as definedin claim 3 wherein said radar signal pulse generating means comprises aK_(a) band IMPATT noise generator producing approximately 1 microsecondpulses having a bandwidth of approximately 500 MHz, said IF circuit hasa bandwidth of approximately 500 MHz and said video circuit has abandwidth of approximately 250 MHz.